With an inverter circuit, when a switching element is turned OFF, a surge voltage is applied to that switching element due to the operation of leakage inductance between the primary side and the secondary side of a transformer, and due to the inductance component of the main circuit. In order to prevent this, a snubber circuit is connected in parallel with the switching element.
In a typical snubber circuit, a snubber capacitor and a snubber resistor for damping which attenuates surge voltage oscillation are connected in series. However with this type of circuit the efficiency is poor, since the charge for charging and discharging the snubber capacitor is converted to heat by the snubber resistor.
In order to eliminate the above described problem with a snubber circuit, a discharge prevention type clamp snubber circuit has been proposed, in which a diode for discharge prevention is connected in series with the snubber capacitor, so that the voltage across the switching element is clamped to the voltage to which the snubber capacitor is charged. Furthermore, along with this circuit, a discharge resistor is provided for conducting a discharge current to the power supply, when the voltage to which the snubber capacitor is charged tries to rise to be higher than this clamp voltage. With this discharge prevention type clamp snubber circuit, the efficiency is not deteriorated due to heat being generated in the snubber resistor, since no snubber resistor is provided. Moreover, since the discharge current which flows in the discharge resistor corresponds only to the amount by which the voltage to which the snubber capacitor is charged is elevated above the clamp voltage, accordingly the heat generation due to the discharge current is not great, provided that this amount of voltage elevation is not very high.
On the other hand, generally well known types of inverter circuit include the full bridge type inverter circuit, the half bridge type inverter circuit, and the center tap push-pull type inverter circuit.
With the full bridge type inverter circuit, the cost is high, since four switching elements are used.
With the half bridge type inverter circuit, while two switching elements are sufficient, the currents which flow in the switching elements and in the primary winding of the transformer are twice as great, as compared with the full bridge type inverter circuit or the center tap type inverter circuit. Due to this, it is impossible to avoid increase in cost, since the switching elements and the transformer are increased in size.
With the center tap push-pull type inverter circuit, two switching elements are sufficient, and moreover the currents which flow in the switching elements and in the primary winding of the transformer are not great, in a similar manner to the case with the full bridge type inverter circuit.
However, with a center tap push-pull type inverter circuit, since the power supply Vin is connected to the center tap of the primary winding P of the transformer, a leakage inductance is present in the combination of the left and right primary windings P. Due to this, the following problem occurs.
Via the leakage inductance described above, the surge voltage which is generated when the first switching element is turned OFF is clamped by a free wheel diode connected to the second switching element. However perfect clamping is not possible, since the leakage inductance described above is present, and there is the problem that the surge voltage described above becomes excessively great.
In the background art described above, a center tap push-pull type inverter circuit which utilizes a discharge prevention type clamp snubber circuit is proposed (refer to Patent Document #1).
With the inverter circuit disclosed in this document, since it is a center tap push-pull type inverter circuit, two switching elements are sufficient, and moreover the currents which flow in the switching elements and in the primary winding of the transformer are not great. Furthermore, since the discharge prevention type clamp snubber circuit is provided it is possible to suppress surge voltage with comparatively high efficiency. Patent Document #1: Japanese Laid-Open Patent Publication 2001-112253.